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Fabrication of uniformly dispersed nanoparticle-doped chalcogenide glassChao Lu, Juliana M. P. Almeida, Nan Yao, and Craig Arnold Citation: Applied Physics Letters 105, 261906 (2014); doi: 10.1063/1.4905283 View online: http://dx.doi.org/10.1063/1.4905283 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mineralization and optical characterization of copper oxide nanoparticles using a high aspect ratio bio-template J. Appl. Phys. 116, 154308 (2014); 10.1063/1.4898809 Effects of surface functional groups on the formation of nanoparticle-protein corona Appl. Phys. Lett. 101, 263701 (2012); 10.1063/1.4772509 Photoexpansion and nano-lenslet formation in amorphous As2S3 thin films by 800 nm femtosecond laserirradiation J. Appl. Phys. 112, 033105 (2012); 10.1063/1.4745021 Effect of cluster size of chalcogenide glass nanocolloidal solutions on the surface morphology of spin-coatedamorphous films J. Appl. Phys. 103, 063511 (2008); 10.1063/1.2895005 Structure and grain growth of Ti O 2 nanoparticles investigated by electron and x-ray diffractions and Ta 181perturbed angular correlations J. Appl. Phys. 100, 024305 (2006); 10.1063/1.2214182
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Fabrication of uniformly dispersed nanoparticle-doped chalcogenide glass
Chao Lu,1,2 Juliana M. P. Almeida,1,3 Nan Yao,2 and Craig Arnold1,2,a)
1Department of Mechanical and Aerospace Engineering, Princeton, New Jersey 08544, USA2Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton,New Jersey 08544, USA3S~ao Carlos Institute of Physics, University of S~ao Paulo, 13560 S~ao Carlos, SP, Brazil
(Received 17 October 2014; accepted 17 December 2014; published online 30 December 2014)
The dispersion of metallic nanoparticles within a chalcogenide glass matrix has the potential for
many important applications in active and passive optical materials. However, the challenge of
particle agglomeration, which can occur during traditional thin film processing, leads to materials
with poor performance. Here, we report on the preparation of a uniformly dispersed
Ag-nanoparticle (Ag NP)/chalcogenide glass heterogeneous material prepared through a combined
laser- and solution-based process. Laser ablation of bulk silver is performed directly within an
arsenic sulfide/propylamine solution resulting in the formation of Ag NPs in solution with an
average particle size of less than 15 nm as determined by dynamic light scattering. The prepared
solutions are fabricated into thin films using standard coating processes and are then analyzed using
energy-dispersive X-ray spectroscopy and transmission electron microscopy to investigate the
particle shape and size distribution. By calculating the nearest neighbor index and standard normal
deviate of the nanoparticle locations inside the films, we verify that a uniformly dispersed
distribution is achieved through this process. VC 2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4905283]
Photodarkening, photobleaching, and photodoping
are well-known light-induced phenomena in chalcogenide
glasses.1–3 The photodoping effect has drawn much attention
recently due to its potential application in various areas, such
as mid-infrared communications, holographic data storage,
diffractive elements, and sensing devices.4–6 However, in
conventional photodoping procedures, which rely on thermal
evaporation and sputtering to create a thin layer of metal on
top of the chalcogenide film, the thickness of the doped layer
is limited by the diffusion depth of the silver.7 Also the
uniformity of the doped layer is difficult to control, thereby
setting limitations in fabricating application-favorable bulk
structures.
One possible method to resolve these difficulties in fab-
ricating chalcogenide films is to employ a solution-based
process.8 In particular, by incorporating nanoparticles into
the deposited films, we can hope to achieve a uniform distri-
bution of silver throughout the entire depth, rather than
having silver concentrated only at the surface. Previous
researchers have shown the ability to distribution quantum
dots into chalcogenide systems using solution-based meth-
ods.9,10 In addition, these approaches can readily realize
large area or large thickness films, and the same solutions
can also be adopted for other precision dispensing techniques
such as mold casting, ink jet or laser direct write, allowing
spatial control over the added material. Nevertheless, uni-
form doping of silver nanoparticles (Ag NPs) into a chalco-
genide glass matrix without agglomeration remains
challenging, due to the tendency of Ag NPs to aggregate.
In this paper, we present experimental results of fabri-
cating uniformly dispersed nanoparticle-doped chalcogenide
glass using laser ablation11,12 and solution processing
methods.13,14 In the fabrication process, we first focus a
pulsed laser beam onto the surface of a bulk metallic sample
within an arsenic sulfide/propylamine solution and ablate the
material. The ejecta expandby carrying out the nanoparticle
generation steps directly in the solution of interest instead of
ideal solvents such as water or ethanol, the laser ablation
methods into the liquid solution, and condenses into in a sus-
pension of silver nanoparticles. In contrast to prior studies
which use ideal and/or pure solvents such as water or etha-
nol, or other organics,15–17 we perform this process directly
in the glass solution of interest. Ag-doped chalcogenide films
are then fabricated by spin-coating the resulting solution.
The prepared solution and films are analyzed using UV-Vis
spectroscopy, dynamic light scattering (DLS), energy-
dispersive X-ray spectroscopy (EDX), and transmission
electron microscopy (TEM) to investigate the particle struc-
ture and distribution. The results demonstrate that this pro-
cess can avoid the aggregation and additional processing
steps associated with other nanoparticles generation techni-
ques, such as wet chemical approaches,18 and a uniformly
dispersed nanoparticle-doped chalcogenide glass has been
fabricated.
Arsenic sulfide (As2S3) solution was prepared by grind-
ing bulk As2S3 pieces into a fine powder and dissolved into
n-propylamine solvent at a concentration of 0.8 mol/l.
Dissolution was carried out inside a sealed glass chamber to
prevent solvent evaporation. The dissolving process usually
took more than three days and a magnetic stirrer was used to
expedite this process. Exposure of solution to atmospheric
moisture was kept to a minimum throughout preparation pro-
cedure since water can lead to precipitate formation.19
To obtain the nanoparticles in As2S3 solution, we used
the experimental setup shown in Fig. 1. A silver target
(>99.99% purity, thickness 5.0 mm) was placed at thea)[email protected]
0003-6951/2014/105(26)/261906/4/$30.00 VC 2014 AIP Publishing LLC105, 261906-1
APPLIED PHYSICS LETTERS 105, 261906 (2014)
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bottom of a Teflon cell, which was filled with As2S3 solution
afterwards. The silver target was covered by a layer of
solution 4 mm deep. The cell was then transferred into an
air-tight vacuum chamber. All these operations were carried
out inside a nitrogen-filled glovebox, to avoid the influence
of oxygen.
The chamber was then placed under the focus of a 30 ps,
1000 Hz, 1064 nm Q-Switched Nd:YAG laser beam, with
maximum energy of 1 mJ/pulse. The Gaussian laser beam
was focused, through the liquid layer, onto the target surface
using a lens with a focal length of 50 mm. Since absorption
by the arsenic sulfide solution was very small at the operat-
ing wavelength, the ablation beam passed through without
significant loss. The position of the target relative to the laser
beam was controlled by an X-Y-Z stage to maintain proper
focus and to avoid deep hole drilling in the target. Ablation
was performed for 10, 25, and 40 min, and we denote the
obtained solutions as 10-, 25-, and 40-min solutions in this
paper, respectively. Ablation was accompanied by the emis-
sion of plasma light from the surface, and bubbling was
observed immediately upon irradiation. The solution gradu-
ally turned brownish-yellow as the concentration of Ag NPs
increased. Finally, the chamber was moved back to the
glovebox, where the solution was collected.
Immediately after synthesis of the solutions, UV-Vis
absorption measurements of silver were conducted using an
Ocean Optics HR4000 high-resolution spectrometer. In order
to characterize morphology in thin films, TEM experiments
were carried out on a Philips CM200 microscope operating
at 200 kV. High resolution images were acquired using a
Gatan Orius 200 camera. Samples for TEM characterization
were prepared by pouring a droplet of the colloidal solution
onto 400-mesh copper grids covered with a holey carbon
film (from Ted Pella). The excess solvent was then allowed
to evaporate.
These suspensions were then deposited onto silicon
wafers for SEM and EDX analysis. Specifically, the solution
was first pipetted onto a silicon substrate, and the substrate
was immediately spun at 1000 rpm for 30 s. Resulting films
were soft-baked under vacuum at 60 �C for 1 h to remove
most of the solvent and then hard-baked at 180 �C to remove
excess solvent and to further densify the glass.20 Films pre-
pared under these conditions typically were approximately
1 lm thick and ready for subsequent analysis.
As photos of the generated Ag NPs solution in Fig. 2
(inset) show the color of these solutions gets noticeably
darker due to the increasing Ag NPs concentration resulting
from higher number of laser shots. Absorption of these solu-
tions is measured by UV-Vis spectroscopy using arsenic
sulfide solution as a control. In the results shown in Fig. 2,
different curves denote spectra from solutions experiencing
different ablation duration. The existence of Ag NPs introdu-
ces surface plasmon resonance bands, but due to the absorp-
tion of the As2S3 solution, only tails of these resonance
bands are observable. Moreover, these measurements dis-
close a redshift behavior of the absorption edge associated
with the increasing concentration of Ag NPs, which agrees
well with the literature.21–23
UV-Vis measurements indicate the existence of Ag NPs
without revealing information about size of those Ag NPs. In
order to determine the size distribution of the Ag NPs, DLS
measurements are performed on the 40-min solution. The
signal from pure As2S3 solution is also acquired for compari-
son. As shown in Fig. 3, background from As2S3 peaks
around 8 nm, which originate from the dissolved As2S3
units.20 The peak from Ag NPs solutions is centered around
14 nm, which reveals the average size of the majority of
nanoparticles. The DLS measurements are also performed on
10- and 25-min solutions, and the same distribution of Ag
NPs are obtained, revealing the size is independent with
ablation time. These values are in good agreement with the
literature,24,25 where nanoparticles average size of 5–25 nm
are generated in ideal solutions like water.
Measurements on the solution phase demonstrate that
the size of the nanoparticles is independent of ablation time,
while the concentration of particles is clearly rising with
increasing ablation time. However, the ultimate test of the
particle distribution is the fabrication of a film with a uni-
form dispersion of nanoparticles. Fig. 4 shows EDX results
FIG. 1. Schematic diagram of the experimental setup.
FIG. 2. Photos (inset) and UV-Vis absorbance spectrum of solution gener-
ated with different ablation duration. As the ablation time increases, the
color of the solutions changes from yellow to brown, indicating the absorp-
tion edge is red-shifted.
261906-2 Lu et al. Appl. Phys. Lett. 105, 261906 (2014)
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from spin-coated films using the 40-min solution. The rela-
tive weight of silver is measured to be 1.3%. Armed with
this value, the concentration of As2S3 solution (2 g/10 ml),
and the average particle size from DLS measurements, the
generation rate of Ag NPs is calculated to be 1.52� 108
nanoparticles formed per pulse, which is equivalent to 7.8
mg/h in terms of the particles weight.
To determine the structure and distribution of the Ag
NPs, we use high-resolution TEM, shown in Fig. 5. The
TEM images (Figs. 5(a) and 5(b)) demonstrate the existence
of Ag NPs and indicate their uniform distribution. Fig. 5(a)
is the dark field TEM image, in which only the diffraction
signal from crystalline structures is collected. Since arsenic
sulfide is in an amorphous state, the bright spots in Fig. 5(a)
are the silver nanoparticles. The diffraction pattern from the
Ag NPs (Fig. 5(c), inset) exhibits hexagonal symmetry that
can be attributed to the basal plane of the hexagonal phase.
This agrees well with the literature where it has been shown
the hexagonal phase of silver stabilizes only in the nanocrys-
talline form for particles less than 30 nm.26 Fig. 5(d) shows a
high resolution TEM image of silver nanoparticles which
demonstrates a high degree of crystalline order. The spacing
between the lattice fringes is measured to be 2.5 A.
The TEM image from Fig. 5(a) is transformed into a
high contrast image, as shown in Fig. 5(b), which more
clearly shows the spacing distribution of nanoparticles. We
quantify uniformity and dispersion of the distribution using
the nearest neighbor index (NNI), a standard method in spa-
tial analysis that is used to determine the degree of spatial
dispersion in a population.27 In general, if the distribution of
the points is clustered together, the average distance between
nearest neighbors will be shorter than if the particles are
scattered throughout the sample. The NNI is defined as the
ratio of the average inter-point distance between nearest
neighbors �d to the expected value of the average inter-point
distance if the sample were randomly dispersed Eð�dÞ.28,29
The value of the NNI can range between the theoretical
extremes of 0 (where all points are at the same location) and
2.1419 (where points have a perfectly uniform distribu-
tion).28 The equations for these parameters are given by
NNI ¼�d
E �dð Þ; �d ¼
Xn
i¼1
di=n; E �dð Þ ¼ 0:5ffiffiffiffiffiffiffiffiA=n
p: (1)
For the data in Fig. 5(b), n¼ 265 is the total number of
particles, obtained using the “particles analysis” utilities
from Image J, and A¼ 2156� 2156 nm2 is the area of the
studied region. di is the distance from ith particle to its
nearest neighbor, determined by measuring all neighboring
distances and taking the minimum. �d is calculated to be
66.68 nm using ImageJ. Eð�dÞ is determined to be 66.21 nm.
The NNI is then readily evaluated to be 1.01, which reveals
a random dispersion.
FIG. 3. DLS data with normalized areas. The measurement data are repre-
sented by the circles, while the trendlines are fitting curves using Gaussian
models. As2S3 solution shows peaks around 8 nm, while the average diame-
ters of the silver particles in the 40-min solution are approximately 14 nm.
FIG. 4. EDX study of the film spin-coated using the 40-min solution. A sili-
con wafer used as the substrate leads to the Si peak. The relative weight of
silver is 1.3%.
FIG. 5. Typical transmission electron micrograph image of Ag NPs gener-
ated by laser ablation of a silver target in As2S3/propylamine solution. (a)
Dark field TEM image of silver nanoparticles uniformly distributed inside
As2S3 film. (b) Inverted high contrast image corresponding to (a). Black dots
are silver nanoparticles. (c) Electron diffraction patterns from a cluster of
Ag NPs of the hexagonal phase with colored lines added for eye guiding. (d)
A high resolution TEM image of silver nanoparticles which demonstrates a
high degree of crystalline order. The spacing between the lattice fringes is
measured to be 2.5 A.
261906-3 Lu et al. Appl. Phys. Lett. 105, 261906 (2014)
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128.112.142.131 On: Wed, 25 Feb 2015 21:24:55
Finally, to test if the calculated NNI is statistically dif-
ferent from that of a random process it is necessary to calcu-
late the standard normal deviate of the distribution (Z) using
Z ¼ ð�d � Eð�dÞÞ=r�d , where r�d ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi0:0683A=n2
pis the stand-
ard deviation of �d .29 If the value of Z is within [�1.96, 1.96]
the distribution of points is considered to be random at the
95% confidence level.29 Z is calculated to be 0.218. The
value of NNI and Z evidences a uniform spatial dispersion of
the particles.
In summary, we report the fabrication of uniformly dis-
persed Ag NP/chalcogenide glass heterogeneous material
prepared through a combined laser- and solution-based
process. We are able to obtain uniform distribution of nano-
particles in both the solution and thin film phases, which is
evidenced by the high resolution TEM measurement and the
NNI analysis. The clustering or agglomeration that is typi-
cally associated with solution based methods in nanopar-
ticles fabrication are avoided through this approach. We
believe the process is applicable to other metals and other
chalcogenide glass solutions.30,31 These materials have a
great potential for applications in diffractive elements and
sensing devices, particularly in cases where thick films uni-
formly doped by Ag NPs are crucial. Furthermore, they have
ability to be photo-responsive, and could be used for direct
writing, as well as recording of optical information, such as
holographic data storage.
We thank Joseph Buttacci for assistance with
experimental setup construction. We thank Gerald Poirier
and Yao-wen Yeh for assistance characterizing samples. We
gratefully acknowledge support by NSF through the
MIRTHE Center (Grant No. EEC-0540832), as well as
support from the Princeton-University of S~ao Paulo
partnership program. J.M.P.A. acknowledges the S~ao Paulo
research foundation for the financial support. N.Y.
acknowledges the partial support of the NSF-MRSEC
program through the Princeton Center for Complex
Materials (DMR-0819860).
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